1. Field of the Invention
This invention relates in general to refrigerant expansion devices for use in thermodynamically reversible compression refrigeration systems having heating and cooling modes of operation. More specifically, this invention relates to a single mechanical expansion device that is capable of operating as a variable area expansion device for both the heating and cooling modes of such a system.
2. Description of the Prior Art
A compression refrigeration system comprises a compressor, a condenser, an expansion device and an evaporator connected in a closed circuit to provide refrigeration. Hot compressed refrigerant vapor from the compressor enters the condenser, where it transfers heat to an external heat exchange medium and condenses. Liquid refrigerant, at a high pressure, flows through the expansion device, where the refrigerant undergoes a pressure drop and at least partially flashes to a vapor. The liquid-vapor mixture then flows through the evaporator where it evaporates and absorbs heat from the external surroundings. The low pressure refrigerant vapor then returns to the compressor to complete the circuit. It has long been recognized that the energy rejected from a refrigeration cycle during condensation may be used to provide heating. Such a system where the flow of refrigerant through the heat exchangers is reversed is commonly referred to as a heat pump.
Typically, to convert the cooling cycle to a heating cycle the duty of the two heat exchangers is thermodynamically reversed. To achieve this result, the direction of refrigerant flow through the system is reversed by changing the connection between the suction and the discharge side of the compressor and the two heat exchangers. This is accomplished for example, by positioning a four-way valve which interconnects the heat exchangers with the inlet and outlet to the compressor. The cooling condenser then functions as an evaporator, while the cooling evaporator serves as a heating condenser. To complete the thermodynamic reversal, the refrigerant must be throttled in the opposite direction between the heat exchangers. Reversible refrigerant cycles have typically used a capillary tube or a double expansion valve and by-pass system positioned in the supply line connecting the two heat exchangers to accomplish throttling in either direction.
Capillary tubes impose serious limitations upon the operational range of a heat pump system in which they are used and accordingly are not frequently employed.
In the double expansion valve arrangement, two opposed expansion valves are positioned in the refrigerant supply line extending between the two heat exchangers. A valve operated by-pass is also positioned parallel to each expansion valve. When the refrigeration cycle is reversed, the by-pass valves are actuated to alternatively utilize one expansion device and by-pass the other.
Commonly assigned U.S. Pat. No. 3,992,898 entitled "Movable Expansion Valve" and issued on Nov. 23, 1976, in the name of Duell, et. al. discloses one approach to eliminating the two expansion valve/two bypass valve arrangement is an expansion device wherein the refrigerant metering port is formed in a free floating piston which is mounted within a chamber. When refrigerant flows through this device in one direction, the free floating piston moves to one position wherein the refrigerant flow is through the metering port thereby serving as an expansion device. When refrigerant flows through this device in the opposite direction, the free floating piston moves to a second position wherein refrigerant is allowed to flow through a number of flow channels formed in the outer peripheral surface of the piston to thereby allow substantially unrestricted flow through the device. This arrangement allows such a device to be used, in combination with a second expansion device of the same design, in a heat pump system to allow the desired expansion of the refrigerant through the system flowing in both the cooling and heating directions. One device is located adjacent to the indoor coil for the cooling mode of operation while the second device is located near the outdoor coil for the heating mode of operation.
In each of the above-described heat pump systems, the system includes two expansion devices, one being dedicated to the cooling mode of operation and the other to the heating mode of operation. Further, each of the expansion devices is of the fixed orifice type wherein a single fixed orifice is selected for each mode of operation which represents a compromise orifice for the wide range of operating conditions which the system may see in each of the modes of operation.
One way of obtaining variable control of the expansion orifice is the use of thermostatic expansion valves. A thermostatic expansion valve controls the flow rate of liquid refrigerant entering the coil serving as an evaporator as a function of the temperature and pressure of the refrigerant gas leaving the evaporator. While being highly efficient in their operation and readily responsive to changes in load upon the system to vary the flow of refrigerant to the evaporator, thermostatic expansion valves are also complicated and expensive. Further, in split system type air conditioning and heat pump systems, where the compressor and condenser are located outside at a remote location from the evaporator, the distance of the sensing bulb from the compressor results in less than optimum conditions in such systems.
It has been recognized that the need exists for a refrigerant expansion device which is inexpensive to manufacture and which is effective in performance over a wide range of operating conditions. One approach to solving this problem has been a refrigerant flow metering device which has a flow metering passage which varies in cross-section in response to changes between the high and low side pressures in the refrigeration system. One such device is described in commonly assigned U.S. Pat. No. 3,659,433 entitled "Refrigeration System Including a Flow Metering Device" issued on May 2, 1972 in the name of David N. Shaw.
One device which provides such a response is a flow metering valve which has a housing with a flow passage in which is mounted a movable piston having a flow metering port extending therethrough. An elongated member within the housing extends into the metering port of the piston. The elongated member and the metering port cooperate to define a flow metering passage between them. The elongated member is configured such that the cross-sectional area varies in relation to the position of the elongated member to the flow metering port. Means are provided for supporting the elongated member within the housing and for controlling the axial position of the elongated member and the piston with respect to one another as a function of the differential pressure across the flow metering piston.
As discussed above in connection with the '898 patent, it is common practice to use two expansion devices in a heat pump system, one dedicated to the cooling mode of operation and the other dedicated to the heating mode.
It has long been an objective to provide a single expansion valve which is capable of providing the expansion function in both the cooling and heating modes of operation of a heat pump system. One approach has been a dual flow electronic expansion valve. One such valve is disclosed in U.S. Pat. No. 4,548,047 entitled "Expansion Valve" issued on Oct. 22, 1985 to Hayashi, et al. This patent describes an expansion valve which has the ability to allow reversible flow of the refrigerant to take place. Typically such valves allows control of the flow rate of the refrigerant regardless of the direction of flow of the refrigerant, so that control may be effected both in the cooling and heating modes by using a single valve. In such devices, typically, input signals are generated by a complex electronic control system which are in turn applied to an electromagnetic coil which controls a plunger which in turn actuates a valve.
Another electronically controlled expansion valve is shown in U.S Pat. No. 4,686,835 entitled "Pulse Controlled Solenoid Valve With Low Ambient Start-up Means", issued on Aug. 18, 1987 to Alsenz. Electronically actuated solenoid flow control valves of the type disclosed in these patents require programmed multi-processor control systems which are extremely expensive. As a result, such control devices are economically attractive in only the most expensive air conditioning/heat pump systems.
The need accordingly exists for a simple, inexpensive, single expansion device that is capable of efficiently controlling a heat pump system in both the heating and cooling modes of operation.